1. Technical Field
The present invention is directed to the art of RF broadcast transmission systems and, more particularly, to improvements in controlling the linearity performance of an inductive output tube (IOT).
2. Description of the Prior Art
It is known that an inductive output tube (IOT) has particular application for use in television broadcasting wherein high kilowatt level RF power is required. An example of such an IOT is found in the U.S. Pat. No. 6,232,721 to D. Danielsons assigned to the same assignee as the invention herein and the disclosure of which is herein incorporated by reference.
Inductive Output Tubes or IOT, as they are commonly called, are high vacuum electron tubes, which allow an electron beam to travel from one end to another in a controlled way. There are four primary parts to an IOT: a cathode which emits electrons, an anode which accelerates the electrons, a collector which collects the electrons, and a grid for controlling the electron emission. The electrons are emitted from a spherical surface cathode consisting of a tungsten matrix heated from behind by a tungsten heater. A spherical pyrolytic carbon grid is positioned close to the cathode and controls the emissions of electrons from the cathode. The cathode is maintained at a relatively high potential (−35,000 volts for typical tubes) while the grid is at a relatively low potential (−50 to −250 volts for typical tubes) with respect to the cathode. If the grid is made less negative with respect to the cathode, then more electrons are emitted. The high electric field between the cathode and anode makes the emitted electrons travel toward the anode or collector. A magnetic field is used to focus the electrons into a beam. Emitted electrons are collected in the collector completing the circuit.
The Inductive Output Tube is used primarily as a high power UHF amplifier. One primary use is in UHF television transmitters operating in the frequency range of 470 MHz to 860 MHz. It is used both for analog television and digital television transmissions. In order to obtain good efficiency, the IOT is operated in a class A/B mode of operation. Due to the class A/B mode of operation, the amplifier draws current which is proportional to the modulation frequencies of the RF signal applied. For analog and digital television signals, these modulation frequencies cover the range of DC through 8 MHz and are commonly called video current.
In the construction of an IOT, the pyrolytic grid is extremely fragile. Due to the high acceleration voltages used, it is possible for the tube to arc from grid to anode. If an arc occurs, the high tension supply may destroy the grid. To overcome this problem, a crowbar or other current limiting device is placed between the IOT and the high tension supply. If an arc occurs, the crowbar directs the high tension supply current away from the IOT preventing the delicate grid from being damaged. Common crowbars use either a gas filled thyratron or a triggered spark gap. These crowbars use a controlled arc to divert the current from the high tension supply away from the IOT. Since the undesired arc in the IOT and the controlled arc in the crowbar have the same impedance, a series resistor must be placed between the crowbar and the IOT, thus forcing the high tension current through the crowbar and away from the IOT.
Reference is now made to FIG. 1 that illustrates a prior art system employing a low pass filter in conjunction with a circuit for controlling an inductive output tube (IOT). The IOT and associated circuits is located in the block that is labeled “L”. The IOT employs an internal grid structure that is very fragile, as noted hereinbefore, and hence, the tube is easily damaged internally if exposed to high voltage arcing. It has been recommended that a transmitter employing an IOT should be able to protect it from damage in the event of such an internal arc. That is, the beam power must be removed from the IOT to limit the energy dissipated within it to less than about 20 joules. This may be achieved by employing the crow bar circuit (CB) shown in FIG. 1.
It has been proposed to employ a fast disconnect system that would be capable of protecting a 300 mm length of thin copper wire having a diameter on the order of 0.1143 to 0.127 mm at voltages above 5 kV. Calculations will show that the energy (I2T) to fuse the wire is approximately 13.6 joules, and this is lower than the 20 joules specified in the statement noted above.
The circuit illustrated in FIG. 1 includes an AC line input voltage supply 10 that supplies an AC voltage to a transformer and rectifier circuit A2 by way of a switch A1. The transformer and rectifier circuit A2 provides a six pulse rectified supply. The ripple voltage may be on the order of −27.5 dB. If the ripple voltage is to be held to a level less than −60 dB, then an additional −32.5 dB of ripple attenuation is needed from the low pass filter F1 which is comprised of inductor L1 and resistor R1 in series with capacitor C1 to ground. This filter is somewhat bulky and stores a significant amount of energy. For example, a typical value for the capacitor C1 is on the order of 8 uF. This capacitor alone at 36 kV has a storage energy on the order of 5,200 joules which is significantly more than what the load device can handle during an internal arcing event.
The filter F1 uses a low pass filter to remove the AC hum and ripple frequency of the linear power supply provided by the transformer and rectifier circuit A2. The lower bandwidth filter provides more ripple rejection and, in turn, more stored energy.
The crowbar circuit CB is a high speed switch that connects the high voltage terminal to ground potential in the event of an internal arc in the load device L. The crowbar circuit is triggered and shorts the high voltage power supply to ground potential. This allows the energy to be dissipated in the circuit path of the crow bar circuit CB instead of the inductive output tube (IOT) load L. While the crowbar circuit is enabled, the input switch A1 is switched off to remove AC power to the transformer and rectifier A2.
Reference is now made to FIG. 2 which illustrates another prior art circuit for controlling and protecting an IOT located in a load L. Since FIGS. 1 and 2 are very similar, like components in both figures are identified with like character references and only the difference are described below in detail. FIG. 2 includes smaller output filter network which includes inductor L1, capacitor C1 and a resistor R2. The energy stored in this filter is optimized so as not to exceed the manufactured recommended level before internal damage to the IOT can take place. This filter has lower energy storage than that in FIG. 1. Because FIG. 2 employs a small, low pass filter, the crow bar circuit of FIG. 1 is removed.
A disadvantage of using a low storage filter configuration as shown in FIG. 2 is that more ripple voltage is evident and performance of the system will be compromised by the residue ripple on the power supply. The low pass filter system is sufficient for a DTV (digital television) signal. The DTV signal has a lower signal to noise (SNR) ripple requirement from the high voltage power supply. This is good for the DTV signal because the residue ripple is not adequate for the traditional analog television transmitter, wherein the analog television transmitter needs a ripple voltage around 60 dB.
In the circuit of FIG. 2, the series resistor R2 provides a current limiting function. If a short circuit is detected by a protection circuit PC, a trigger signal is sent to the switch A1 to open the switch and remove the supply 10.
It is to be noted that in the prior art of FIGS. 1 and 2, low pass filter F1 or F2 is employed. These are relatively large and expensive components. The present invention contemplates achieving control of an IOT in the load 12 without employing such low pass filters. The control is directed toward minimizing the AC ripple voltage by obtaining a smooth DC voltage and without employing a large low pass filter. FIGS. 1 and 2 of the prior art are shown as FIGS. 1 and 2 in the U.S. patent to A. B. See et al. 6,724,153.